The present invention relates to a plasma ion source mass spectrometer and in particular to a plasma ion source mass spectrometer provided with a means suitable for quenching background ions which interfere with metal ions, quenching of such background ions being important for practical use thereof.
In conventional plasma ion source mass spectrometers, argon gas or nitrogen gas is used as a plasma gas and ions are produced by inductively coupled plasma (ICP) or microwave induced plasma (MIP), which are introduced into mass spectrometer and subjected to mass spectrometeric analysis. These devices are disclosed, for example, in "Bunseki (Analysis)", 1987, 7 (1987), pp. 480-484, "Anal. Chem.", Vol. 52, (1980), pp. 2283-2289, "Anal. Chem", Vol. 59, (1987), pp. 1664-1670, and Japanese Patent Application Kokai (Laid-Open) No. 62-219452.
Outline of plasma ion source mass spectrometers [ICP mass spectrometer (ICPMS) and MIP mass spectrometer (MIPMS)]is shown in FIG. 9. The object of them is normally to analyse ultra trace elements in a solid sample. The sample is dissolved in an acid or an organic solvent, the resulting liquid sample is fed to a nebulizer and thus nebulized sample is introduced into ionizing part 1 with a carrier gas such as argon or nitrogen. Plasma (ICP or MIP) is formed in plasma generating part 2 in the ionizing part 1 and the introduced sample is ionized in this plasma. Pressure in the plasma generating part is 1 atm. The ions produced in the plasma are introduced into mass analyzing part 5 of high vacuum through differential pumping regions 3,4 and separated according to mass to charge ratio (m/z, m: mass of ions and z: valency of ions) and then detected.
The above-mentioned conventional techniques give no consideration to quenching of background ions which are produced in plasma and which interfere with metal ions and this is a great problem for putting to practical use the ICP mass spectrometer (ICPMS) or MIP mass spectrometer (MIPMS). That is, in ICPMS which uses argon gas as a plasma gas, ions originating from argon as a main component and from nitrogen as an impurity and from acid and water used for making a sample in the form of aqueous solution which are introduced into ion source are produced as principal ions.
Amounts of these argon, nitrogen, acid and water introduced into ion source are much more than the amount of the trace elements to be analyzed in the sample simultaneously introduced into the ion source. Therefore, with reference to the ions produced in plasma, amount of ions originating from argon, nitrogen, acid and water is also much more than that of the ions of elements to be analyzed. Examples of ions originating from argon, nitrogen, acid and water are shown in Table 1 as background ions. There are many kinds of these background ions.
TABLE 1 ______________________________________ m/z Background ions Interfered elements ______________________________________ 28 N.sup.2+, CO.sup.+ .sup.28 Si (.sup.27 Al) 29 N.sub.2 H.sup.+, COH+, N.sup.15 N.sup.+ .sup.29 Si 30 NO.sup.+ .sup.30 Si 31 NOH.sup.+, .sup.15 NO.sup.+ .sup.31 P 32 O.sup.2+ .sup.32 S 33 O.sup.17 O.sup.+ .sup.33 S 34 .sup.17 O.sub.2.sup.+, O.sup.18 O.sup.+ .sup.34 S 40 Ar.sup.+ .sup.40 Ca (.sup.39 K) 41 ArH.sup.+ .sup.41 K 48 SO.sup.+ .sup.48 Ti 51 .sup.35 ClO.sup.+ .sup.51 V 52 .sup.35 ClOH.sup.+ .sup.52 Cr 53 .sup.37 ClO.sup.+ .sup.53 Cr 54 ArN.sup.+ .sup.54 Cr .sup.54 Fe 56 ArO.sup.+, N.sup.4+ .sup.56 Fe 57 ArOH.sup.+ .sup.57 Fe 64 SO.sub.2.sup.+ .sup.64 Zn 65 SO.sub.2 H.sup.+ .sup.65 Cu 68 ArN.sub.2.sup.+ .sup.68 Zn 72 ArS.sup.+ .sup.72 Ge 75 Ar.sup.35 Cl.sup.+ .sup.75 As 77 Ar.sup.37 Cl.sup.+ .sup.77 Se 80 Ar.sup.2+ .sup.80 Se ______________________________________
These ions are those which do not originate from the sample and they are background ions. According to the conventional apparatuses, background ions and sample ions are introduced in admixture into the mass analyzing part and are subjected to mass-separation. Therefore, when background ions and sample ions have the same mass to charge ratio, the peak appearing at the position of that mass to charge ratio includes both the background ion peak and sample ion peak. Besides, since amount of the background ions is much more than that of sample ions, the appearing peaks are mostly for the background ions and considerably interfere with the sample ion peak and measurement becomes impossible. For example, if the element to be analyzed is Ca of mass to charge ratio (m/z) =40 as shown in Table 1, Ca.sup.+ peak overlaps Ar.sup.+ peak which appears at the same m/z and in addition, since Ar.sup.+ peak is extremely higher than Ca.sup.+ peak, the peak appearing at m/z =40 is mostly for Ar.sup.+ ion as background ion and Ca to be analyzed cannot be detected. As shown in Table 1, there are many elements with which the background ions interfere. ICPMS is an analytical device having high detection sensitivity, but has the severe practical problem of the interference.
Moreover, excited molecule produced in plasma is a neutral particle and hence is not mass-separated in the mass analyzing part and reaches an electron multiplier. This excited molecule generates electron in the electron multiplier to cause production of noise. Thus, presence of the excited molecule is a serious obstacle to enhancement of sensitivity of plasma ion source mass spectrometer.
This is the same for MIPMS.